1. Field of the Invention
The present invention relates to the design of filter circuits in integrated circuits, and more specifically to a method and apparatus to enhance flexibility of a low distortion filter circuit to provide variable gain amplification (VGA).
2. Related Art
A filter circuit generally refers to a circuit which selectively passes frequencies of interest, while inhibiting/blocking other frequencies of an input signal. Filter circuits are also often designed to perform an amplification operation, and provide an amplified output signal containing the frequencies of interest. Such amplification is often required, for example, due to the low strength with which signals are often received and thus to take advantage of full swing possible on an output path, as described below in further detail. Integrating the amplification operation in the filter itself (as compared to using a separate amplification stage) results in lower area power.
The amplification operation to be performed in filer circuits often needs to be designed to provide with variable gain amplification (VGA) as well. VGA generally implies that the amplification factor (output voltage level divided by input voltage level) needs to be different at different instances of time. VGA often enables an input signal (or components containing the frequencies of interest) to be amplified to a desired voltage level, when the input signal strength is changing with time. Such a feature may be used, for example, when the output signal (on an output path) of the filter circuit is sampled by an analog to digital converter (ADC), and it may be desirable to use the entire range of input voltages possible at the input of the ADC irrespective of the strength of the input signal.
The ability to vary the gain/amplification is often attained by having components with programmable values (e.g., ability to set the resistance value for a resistor). The programmability may be achieved by connecting multiple sub-components (typically in series or parallel), and making only some of the sub-components active or operational by the use of appropriate switches. In general, a switch is kept in one of open or closed state to make the corresponding sub-component operational, and in the other state of disconnect (or make non-operational) the sub-component. The active sub-components together form the component with a corresponding value.
It is often desirable that flexibility exist to obtain a desired value for components (each component containing one or more sub-components) being used to perform the filtering and amplification tasks. The flexibility is particularly important because a change of value of one component for one characteristic (for example to attain a desired amplification factor) may alter another characteristic (various attributes of the transfer function, such as corner frequency, Q-factor, notch frequency, etc.. As a result, the magnitudes of a number of components in the circuit would be dependent on each other due to such constraints posed by the filter circuit characteristics. The number of components that can be altered independently, without affecting the transfer characteristics, may be referred to as ‘degrees of freedom’.
Filter circuits generally need to be implemented to provide low distortion, in addition to providing VGA and desired transfer function. It is typically desirable that a filter provide a linear response when the amplification factor is set to a constant value, and deviations from such a linear response is referred to as distortion. It is generally desirable to reduce/eliminate distortion since the output signal then would accurately represent the information in the input signals.
What is therefore needed is a filter circuit which provides enhanced flexibility such that a desired transfer function, desired amplification factor, and low distortion can be obtained.
In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.